It is known that a Solid Electrolyte Interface (SEI) coating (hereinafter referred to as a coating) is present on the surface of a negative electrode active material included in a nonaqueous electrode secondary battery. This coating is formed due to the reductive decomposition of an electrolyte on a negative electrode at the time of an initial charge and discharge. The subsequent reaction between the electrolyte and the negative electrode is suppressed by the formation of the coating.
Recently, a nonaqueous electrolyte secondary battery using lithium titanate as a negative electrode active material has been developed. A lithium ion inserted/released potential inherent to the negative electrode in the nonaqueous electrolyte secondary battery is higher than that inherent to the negative electrode used in the conventional batteries. Therefore, the coating is hardly formed on the surface of the negative electrode active material. In such a battery, the decomposition reaction of the electrolyte is not suppressed because of the insufficient formation of the coating. In particular, when the battery is stored in a state close to a full charge at a high temperature, the decomposition reaction of the electrolyte is increased. Thus, there is a problem that gas generation due to the decomposition of the electrolyte is increased, resulting in the expansion of the battery. There is also a problem that the capacity retention ratio due to the rise of the internal resistance is decreased.